1. Technical Field
The disclosure relates to a microelectromechanical filter used for a wireless communication system.
2. Description of Related Art
In a communication system, an acoustic wave filter can be used for extracting a signal to be transmitted and filtering the noises, which is somehow an essential component in the communication system. Regarding a given frequency response, a transfer function of the filter is determined by a quality factor (which is referred to as a Q-value hereinafter). A device loss of a conventional radio frequency (RF) device is increased as a frequency is increased since conductors and media therein are operated under a frequency of GHz. Therefore, a film bulk acoustic wave resonator designed based on structural resonance is gradually used to replace the related device due to its small size, which becomes a main device of a filter used in a mobile phone. If “channel selection filters” are required to satisfy demands of next generation communication systems, the Q-value of the resonator at 1 GHz is required to be higher.
Some conventional resonators have a problem that increasing of the Q-value and decreasing of the impedance cannot be simultaneously achieved, so that a filter with an extremely high Q-value and an impedance low enough cannot be designed. Therefore, it is an important issue to design a filter with an extremely high Q-value and an impedance on demand to match the impedance of the communication system, so as to implement a channel selection filter.